One nasty surprise about fitness trackers is that some people find they irritate their skin. Some get an itchy wrist, or a full-on rash break-out.
There are two causes. Either the strap will be trapping moisture, and the extra friction and wetness will get your wrist narked, or you might get a flare-up of contact dermatitis from something you have an allergy to in the strap – nickel seems to be the most common offender.
Apparently even the Apple Watch wasn't fully tested for this kind of allergic reaction. It may not be exciting, but this one's important. No nickel, please.